The present invention relates to the production of A-Zeolite, and more particularly to a method for the neutralization of A-Zeolite obtained in synthesis plants.
According to the known manufacturing processes, A-Zeolite is synthetized in an aqueous environment of free NaOH. At the end of the reaction an excess of NaOH remains, the presence of which is necessary during the reaction step in order to complete the crystallization process.
In order that A-Zeolite may be handled and used in detergents, same should be deprived from the alkalinity in excess.
According to the known technologies, this operation is performed by washing with demineralized water the filter press or centrifugation cakes up to complete NaOH removal.
The disadvantages of the above procedure are manyfolds. One of these is due to the fact that great energy requirements are requested in the filtration and/or centrifugation step.
Another derives from the necessity of recovering or of treating large amounts of cake washing water, of low NaOH content.
Moreover, after drying, aggregate powders having low apparent density are obtained, which are difficult to be handled, and which form deposits on the fabrics washed with the detergents containing the said Zeolites. It has been proposed to simplify the above process by separating by filtration and/or centrifugation the raw Zeolite from the major amount of the still concentrated mother liquor, without washing it, or by washing it only partially, and by neutralizing with a suitable acid the residual NaOH which impregnates the cake.
The above solution presents however practical difficulties due to the extreme instability of the A-Zeolite in a strong acid environment, so that the neutralization must be performed avoiding the accumulation of neutralizing agents in order as not to fall below (at any point of the system) of a safety pH value (minumum pH value=10).
In order to do so it is necessary to assure the perfect dispersion of the acid throughout the whole mass of Zeolite under treatment.
According to a known prior art process, which is the object of the Japanese Pat. No. 80/27818 in the name of Fuji Davison Kagaku K.K., the Zeolites for detergency are prepared by pulverization of synthetic Zeolites in a fluid energy mill with concurrent neutralization of the said Zeolites by means of sulphuric acid which is sprayed in said mill.
The obtained final products have a water content of from 22.6 to 29.8% and a pH (aqueous solution at 1% of the slurry) of 10.4-11.2.
The above process in which sulphuric acid sprays are used, requests substantially critical working conditions.
It has also been proposed, according to another prior art process described in the German patent application No. 2.412.838, to treat a slurry containing Zeolites and other components of a detergent, with carbon dioxide in order to transform the excess of alkali into sodium carbonate. However, the said process based on the use of a weak acid as neutralizer has the disadvantage of the presence of the sodium carbonate in the final product, which gives rise to problems with the current detergent formulations.
Finally, according to the French patent application No. 2.499.123 on the name of Colgate Palmolive, it has been proposed to utilize, as neutralizing substance for the excess of NaOH present on the particles of A-Zeolite, tridecyl benzenesulphonic acid. The said process is however very expensive, due to the very large amount of tridecyl benzenesulphonic acid requested for the neutralization step.